The field relates to motor vehicle ignition locks.
A common type of motor vehicle ignition lock is a key-operated ignition switch system has a cylinder lock with a barrel and a slot along the long axis of the barrel through and into which a mechanical key inserts. If the key is valid, the pattern that has been cut into an edge of its shaft to create the key's teeth sets the lock tumblers to free them from locking engagement with the barrel. This allows the barrel to rotate when the base of the key is turned. The barrel can then be rotated from the Lock position between the various ignition switch positions including to Accessory, On (Ignition), and Start positions to connect various branch circuits of the vehicle's electrical system to the vehicle's electric power supply as well as back to the OFF position.
The ignition switch allows a connection to be established from the battery to a starter motor for cranking the starter motor to start an engine. It also controls delivery of power to a vehicle system bus independently of its use in starting the vehicle engine. This prevents inadvertent energization of circuits and prevents accessories connected to a vehicle system bus from running down the battery when the vehicle is parked for an extended period of time with an accessory switch left closed.
This ignition lock system has however proven vulnerable to car thieves. A skilled thief can often “hot wire” an automobile or truck in manner of a few seconds. Typically, the key and cylinder lock is either bypassed or pulled in order to carry out a theft. To combat this automotive manufacturers have incorporated a variety of vehicle anti-tampering systems. U.S. Pat. No. 5,186,031 for example provided an electronic interlock system which was constructed to be disabled when the lock cylinder was forcibly removed from its sleeve, which is a common initial action in attempting an unauthorized start of a vehicle. The '031 patent provided an interlock circuit part of which was disposed on a frangible base which was destructed upon forcible removal of the lock cylinder from its sleeve.